Medium- and high-power power converters, such as DC-DC voltage converters, have a variety of applications, including in the railway, automotive, telecommunication and aeronautical industries. For example, in electric railway applications, a wayside/onboard energy storage system may be used to supply power during train departures and absorb excess power produced through regenerative braking. The energy storage system includes a voltage converter disposed between the traction line of the railway and a storage element (e.g., an ultra-capacitor bank). In the telecommunication industry, the network infrastructure may rely at least in part of on batteries to maintain service continuity in the event of voltage surges, sags or brownouts, which calls for the use of voltage converters. In the utility industry, critical equipment and installations are protected against momentary power losses by an arrangement of batteries and voltage converters. In hybrid vehicles, a bi-directional voltage converter is disposed between a car battery and an ultra-capacitor bank in order to charge the battery during regular operation, while replenishing the ultra-capacitor bank during regenerative braking periods.
A voltage converter typically has two sides, one for a lower voltage and one for a higher voltage, and converts between the two. A voltage converter that converts a high voltage input to a low voltage output is said to operate in buck mode, while a converter that converts a low voltage input to a high voltage output is said to operate in boost mode. In some cases, the converter may be bi-directional, meaning that the converter operates in buck mode when the current flows in one direction, while at times of reverse current flow it operates in boost mode.
Typically, a voltage converter utilizes switching elements that are turned on and off in accordance with a specific switching pattern so as to produce a desired output voltage level, which can be at the high- or low-voltage side, depending on whether the converter is operating in boost or buck mode, respectively. The switching pattern is characterized by a duty cycle and a switching frequency, among possibly other parameters.
A conventional switching pattern may produce “hard switching” of the switching elements, which refers to the fact that a switching element is turned off while there is a high current flowing through it or is turned on while there is a high voltage across it. This leads to significant power loss and severe heat dissipation, which requires the converter to be dimensioned and constructed accordingly. Also, switching under such conditions may damage the switching elements. The net result is a bulky system with limited efficiency, high cost and reduced lifespan.
Thus, there is a need in the industry for an improved way in which to control the switching pattern of switching elements in a voltage converter.